FRESHWATER INVERTEBRATE DISTRIBUTION 225 



notably among the stream snails (Pleuroceridae) . Freshwater 

 shrimps in the Family Atyidae, restricted to a few California coastal 

 streams, present a further example. 



Perhaps it should be pointed out that the West appears to be 

 lacking in extensive limestone caves and associated underground 

 stream systems of the sort found in Kentucky, Tennessee, Indiana, 

 Illinois, Missouri, and Florida. Such habitats are often marked by 

 unusual endemic aquatic invertebrates. Interesting counterparts, 

 however, are the occasional western lava-tube caves where a few 

 unusual forms have been found, e.g., Kenkia (turbellarian) and some 

 species of Stygobromiis (amphipods). On the other hand, it should 

 be noted that the West, so far as we know, is lacking in truly archaic 

 species, such as amphipods, isopods, Bathynella, Troglochaetus, 

 and Marifiigia, that are so typical of southeastern Europe. 



5. Unlike the situation for certain vertebrates and for inverte- 

 brate terrestrial groups, it is quite clear that the western freshwater 

 invertebrate fauna involves a zoogeographic situation that is con- 

 fusing, poorly known, and greatly diverse. Omitting insects and a 

 few of the minor taxa, however, an assessment or appraisal of this 

 fauna would seem to indicate five zoogeographic categories, as 

 follows. 



Six taxa are characterized in the West by numerous cosmopolitan 

 species, most of which produce resting eggs, cysts, and other resis- 

 tant and dispersal devices that are remarkably effective in sur- 

 mounting barriers. Protozoa and Rotatoria are typical of this 

 category and are fairly well known in the West. Four other groups 

 (Oligochaeta, Tardigrada, Gastrotricha, and Nematoda) are poorly 

 known, but judging from studies in other parts of the world, we 

 assume that many common species are widely distributed through- 

 out the West. 



A second category includes five taxa that are not necessarily 

 cosmopolitan but whose speciation and abundance in the West 

 are largely unknown. There are a few records of triclad Turbellaria 

 from the West, but little is known about the rhabdocoels, and we 

 are greatly in need of studies on pond, alpine, and springbrook 

 species in this order. Little is known about the Hirudinea of the 

 West; a good many eastern species, however, have highly restricted 

 distributions, and a comparable situation may exist in the West. 

 Of the thirty species of freshwater Porifera in the United States, 



